This invention relates to servo control systems for use in electromechanical devices of the type employing a rotary print wheel, sometimes termed a "daisy wheel printer", wherein the print wheel is both rotated and translated by a pair of motors under control of an electronic control system. More particularly, this invention relates to an improved desired velocity signal generation technique for such servo control systems.
Rotary printing systems having servo control are known in which the rotary print wheel is mounted on a carriage for translation across the width of the print throat area during printing. The carriage is typically translated in right and left directions through a series of cables and pulleys driven incrementally by a DC motor controlled by a suitable electronic control system. As the carriage-wheel assembly is translated from character position to character position along the print line, the print wheel is rotated so that the character pad bearing the next character to be printed is aligned with the striking end of a print hammer, also mounted on the carriage, when the carriage is momentarily decelerated to a rest position. When the carriage has reached the rest condition, the print hammer is actuated to impress the character borne by the pad against an inking ribbon and the face of the printing media to print that character. After the print hammer rebounds, the carriage is translated to the next character location, the print wheel is rotated so that the proper character pad is aligned with the print hammer, and the next character is printed. This process continues until a complete line has been so printed, after which the carriage motion is reversed to print the next line of characters in reverse order, or the carriage is returned to the left-most starting position in preparation for printing the next line of characters.
Proper operation of such rotary printing systems requires an accurate servo control system for both the print wheel and the carriage. The servo system most typically used is a dual mode system having a velocity mode and a position mode. In the velocity mode, the velocity of the controlled element (i.e., print wheel or carriage) is controlled in accordance with a pre-selected velocity profile to ensure optimum initial acceleration to a maximum design velocity value, followed by stepped deceleration until the desired linear or rotary position is achieved. Once this position (frequently termed the HOME position) has been reached, the servo is switched to a position mode of operation in which the linear or angular position of the controlled element is maintained substantially constant. In both modes of operation, position feedback signals generated by a position encoder associated to the controlled element (typically an optical encoder for generating sinusoidal position signals) are used to provide the necessary feedback information specifying the instantaneous position of the controlled element. These signals, either in their pure sinusoidal form or in logically processed pulse form, are coupled to a control unit in which the position information is used to determine certain key parameters, such as direction of print wheel rotation, incremental linear or angular distance from the present position to the next desired position, actual velocity of the controlled element, required incremental velocity, and the like. In addition, the control unit supervises the operating mode of each servo system, i.e., whether velocity or position mode, and generates the necessary servo control signals for switching the operation between the two distinct modes. In addition, when in the position mode the servo system uses one of the analog position feedback signals to monitor excursions of the controlled element away from the desired HOME position in order to generate corrective position signals to the motor driving the controlled element in order to counteract any such deviations.
Critical to the proper operation of dual mode servo systems used in rotary printing systems is the manner in which the velocity control signals are generated when the servo is operated in the velocity mode. Typically, the preselected velocity profile used to establish the desired optimum motion of the controlled element is stored in the form of digital characters of prescribed value in a read only memory (ROM) arranged as a velocity lookup table. Depending on the present angular or linear distance of the controlled element from the required printing position specified by the text information supplied to the control unit, a given digital desired velocity character is accessed from the ROM and coupled to a digital to analog converter (DAC), and the digital multibit character is converted to a corresponding analog signal. This analog signal is then typically compared with a corresponding analog signal representing the actual velocity of the controlled element in order to generate a correction signal, which is supplied to the motor driver and used to control the amount of current supplied to the driving motor for the corresponding controlled element. In known systems, the DAC is a unidirectional or unipolar DAC which generates a unidirectional analog output current which varies in magnitude over a prescribed range, but which is incapable of bidirectional flow. Since the correction signal must be capable of specifying not only magnitude but also direction (i.e., clockwise or counterclockwise for the print wheel and left-to-right or right-to-left for the carriage), the unidirectional analog current output of the DAC must be further processed to provide a bidirectional range of values in order to provide the requisite range of control. This is usually accomplished by providing a range converter circuit between the output of the DAC and the summing junction for the two signals. Since the range converter circuitry is necessarily composed of analog elements, such as a plurality of resistors and an operational amplifier, static errors are introduced into the analog desired velocity signal by virtue of the normal variation of analog parameters in the elements employed. While the introduced error can be somewhat mitigated by using high precision components, this error cannot be eliminated altogether. This problem is exacerbated by the fact that the parametric values of the analog components change with temperature, humidity and time, so that dynamic errors, as well as static errors, change the absolute value of the desired analog velocity signal in an undesirable, and frequently unpredictable, way.